1 use std::cell::RefCell;
2 use std::collections::HashSet;
5 use std::num::NonZeroU64;
7 use rustc::ty::{self, layout::Size};
8 use rustc::hir::{MutMutable, MutImmutable};
9 use rustc::mir::RetagKind;
12 EvalResult, InterpError, MiriEvalContext, HelpersEvalContextExt, Evaluator, MutValueVisitor,
13 MemoryKind, MiriMemoryKind, RangeMap, Allocation, AllocationExtra,
14 Pointer, Immediate, ImmTy, PlaceTy, MPlaceTy,
17 pub type PtrId = NonZeroU64;
18 pub type CallId = NonZeroU64;
20 /// Tracking pointer provenance
21 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
27 impl fmt::Display for Tag {
28 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
30 Tag::Tagged(id) => write!(f, "{}", id),
31 Tag::Untagged => write!(f, "<untagged>"),
36 /// Indicates which permission is granted (by this item to some pointers)
37 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
39 /// Grants unique mutable access.
41 /// Grants shared mutable access.
43 /// Greants shared read-only access.
47 /// An item in the per-location borrow stack.
48 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
50 /// The permission this item grants.
52 /// The pointers the permission is granted to.
54 /// An optional protector, ensuring the item cannot get popped until `CallId` is over.
55 protector: Option<CallId>,
58 impl fmt::Display for Item {
59 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
60 write!(f, "[{:?} for {}", self.perm, self.tag)?;
61 if let Some(call) = self.protector {
62 write!(f, " (call {})", call)?;
69 /// Extra per-location state.
70 #[derive(Clone, Debug, PartialEq, Eq)]
72 /// Used *mostly* as a stack; never empty.
74 /// * Above a `SharedReadOnly` there can only be more `SharedReadOnly`.
75 /// * Except for `Untagged`, no tag occurs in the stack more than once.
80 /// Extra per-allocation state.
81 #[derive(Clone, Debug)]
83 // Even reading memory can have effects on the stack, so we need a `RefCell` here.
84 stacks: RefCell<RangeMap<Stack>>,
85 // Pointer to global state
89 /// Extra global state, available to the memory access hooks.
91 pub struct GlobalState {
94 active_calls: HashSet<CallId>,
96 pub type MemoryState = Rc<RefCell<GlobalState>>;
98 /// Indicates which kind of access is being performed.
99 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
100 pub enum AccessKind {
105 impl fmt::Display for AccessKind {
106 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
108 AccessKind::Read => write!(f, "read"),
109 AccessKind::Write => write!(f, "write"),
114 /// Indicates which kind of reference is being created.
115 /// Used by high-level `reborrow` to compute which permissions to grant to the
117 #[derive(Copy, Clone, Debug, Hash, PartialEq, Eq)]
119 /// `&mut` and `Box`.
120 Unique { two_phase: bool },
121 /// `&` with or without interior mutability.
123 /// `*mut`/`*const` (raw pointers).
124 Raw { mutable: bool },
127 impl fmt::Display for RefKind {
128 fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
130 RefKind::Unique { two_phase: false } => write!(f, "unique"),
131 RefKind::Unique { two_phase: true } => write!(f, "unique (two-phase)"),
132 RefKind::Shared => write!(f, "shared"),
133 RefKind::Raw { mutable: true } => write!(f, "raw (mutable)"),
134 RefKind::Raw { mutable: false } => write!(f, "raw (constant)"),
139 /// Utilities for initialization and ID generation
140 impl Default for GlobalState {
141 fn default() -> Self {
143 next_ptr_id: NonZeroU64::new(1).unwrap(),
144 next_call_id: NonZeroU64::new(1).unwrap(),
145 active_calls: HashSet::default(),
151 pub fn new_ptr(&mut self) -> PtrId {
152 let id = self.next_ptr_id;
153 self.next_ptr_id = NonZeroU64::new(id.get() + 1).unwrap();
157 pub fn new_call(&mut self) -> CallId {
158 let id = self.next_call_id;
159 trace!("new_call: Assigning ID {}", id);
160 self.active_calls.insert(id);
161 self.next_call_id = NonZeroU64::new(id.get() + 1).unwrap();
165 pub fn end_call(&mut self, id: CallId) {
166 assert!(self.active_calls.remove(&id));
169 fn is_active(&self, id: CallId) -> bool {
170 self.active_calls.contains(&id)
174 // # Stacked Borrows Core Begin
176 /// We need to make at least the following things true:
178 /// U1: After creating a `Uniq`, it is at the top.
179 /// U2: If the top is `Uniq`, accesses must be through that `Uniq` or remove it it.
180 /// U3: If an access happens with a `Uniq`, it requires the `Uniq` to be in the stack.
182 /// F1: After creating a `&`, the parts outside `UnsafeCell` have our `SharedReadOnly` on top.
183 /// F2: If a write access happens, it pops the `SharedReadOnly`. This has three pieces:
184 /// F2a: If a write happens granted by an item below our `SharedReadOnly`, the `SharedReadOnly`
186 /// F2b: No `SharedReadWrite` or `Unique` will ever be added on top of our `SharedReadOnly`.
187 /// F3: If an access happens with an `&` outside `UnsafeCell`,
188 /// it requires the `SharedReadOnly` to still be in the stack.
190 impl Default for Tag {
192 fn default() -> Tag {
198 /// Core relation on `Permission` to define which accesses are allowed
200 /// This defines for a given permission, whether it permits the given kind of access.
201 fn grants(self, access: AccessKind) -> bool {
202 // All items grant read access, and except for SharedReadOnly they grant write access.
203 access == AccessKind::Read || self != Permission::SharedReadOnly
207 /// Core per-location operations: access, dealloc, reborrow.
209 /// Find the item granting the given kind of access to the given tag, and return where
210 /// it is on the stack.
211 fn find_granting(&self, access: AccessKind, tag: Tag) -> Option<usize> {
213 .enumerate() // we also need to know *where* in the stack
214 .rev() // search top-to-bottom
215 // Return permission of first item that grants access.
216 // We require a permission with the right tag, ensuring U3 and F3.
217 .find_map(|(idx, item)|
218 if tag == item.tag && item.perm.grants(access) {
226 /// Find the first write-incompatible item above the given one --
227 /// i.e, find the heigh to which the stack will be truncated when writing to `granting`.
228 fn find_first_write_incompaible(&self, granting: usize) -> usize {
229 let perm = self.borrows[granting].perm;
231 Permission::SharedReadOnly =>
232 bug!("Cannot use SharedReadOnly for writing"),
233 Permission::Unique =>
234 // On a write, everything above us is incompatible.
236 Permission::SharedReadWrite => {
237 // The SharedReadWrite *just* above us are compatible, to skip those.
238 let mut idx = granting+1;
239 while let Some(item) = self.borrows.get(idx) {
240 if item.perm == Permission::SharedReadWrite {
244 // Found first incompatible!
253 /// Remove the given item, enforcing barriers.
254 /// `tag` is just used for the error message.
255 fn remove(&mut self, idx: usize, tag: Option<Tag>, global: &GlobalState) -> EvalResult<'tcx> {
256 let item = self.borrows.remove(idx);
257 if let Some(call) = item.protector {
258 if global.is_active(call) {
259 if let Some(tag) = tag {
260 return err!(MachineError(format!(
261 "not granting access to tag {} because incompatible item is protected: {}",
265 return err!(MachineError(format!(
266 "deallocating while item is protected: {}", item
271 trace!("access: removing item {}", item);
275 /// Test if a memory `access` using pointer tagged `tag` is granted.
276 /// If yes, return the index of the item that granted it.
281 global: &GlobalState,
282 ) -> EvalResult<'tcx> {
283 // Two main steps: Find granting item, remove incompatible items above.
285 // Step 1: Find granting item.
286 let granting_idx = self.find_granting(access, tag)
287 .ok_or_else(|| InterpError::MachineError(format!(
288 "no item granting {} access to tag {} found in borrow stack",
292 // Step 2: Remove incompatible items above them. Make sure we do not remove protected
293 // items. Behavior differs for reads and writes.
294 if access == AccessKind::Write {
295 // Remove everything above the write-compatible items, like a proper stack. This makes sure read-only and unique
296 // pointers become invalid on write accesses (ensures F2a, and ensures U2 for write accesses).
297 let first_incompatible_idx = self.find_first_write_incompaible(granting_idx);
298 for idx in (first_incompatible_idx..self.borrows.len()).rev() {
299 self.remove(idx, Some(tag), global)?;
302 // On a read, remove all `Unique` above the granting item. This ensures U2 for read accesses.
303 // The reason this is not following the stack discipline is that in
304 // `let raw = &mut *x as *mut _; let _val = *x;`, the second statement
305 // would pop the `Unique` from the reborrow of the first statement, and subsequently also pop the
306 // `SharedReadWrite` for `raw`.
307 // This pattern occurs a lot in the standard library: create a raw pointer, then also create a shared
308 // reference and use that.
309 for idx in (granting_idx+1 .. self.borrows.len()).rev() {
310 if self.borrows[idx].perm == Permission::Unique {
311 self.remove(idx, Some(tag), global)?;
320 /// Deallocate a location: Like a write access, but also there must be no
321 /// active protectors at all because we will remove all items.
325 global: &GlobalState,
326 ) -> EvalResult<'tcx> {
327 // Step 1: Find granting item.
328 self.find_granting(AccessKind::Write, tag)
329 .ok_or_else(|| InterpError::MachineError(format!(
330 "no item granting write access for deallocation to tag {} found in borrow stack",
334 // Step 2: Remove all items. Also checks for protectors.
335 for idx in (0..self.borrows.len()).rev() {
336 self.remove(idx, None, global)?;
342 /// `reborrow` helper function: test that the stack invariants are still maintained.
343 fn test_invariants(&self) {
344 let mut saw_shared_read_only = false;
345 for item in self.borrows.iter() {
347 Permission::SharedReadOnly => {
348 saw_shared_read_only = true;
350 // Otherwise, if we saw one before, that's a bug.
351 perm if saw_shared_read_only => {
352 bug!("Found {:?} on top of a SharedReadOnly!", perm);
359 /// Derived a new pointer from one with the given tag.
360 /// `weak` controls whether this operation is weak or strong: weak granting does not act as
361 /// an access, and they add the new item directly on top of the one it is derived
362 /// from instead of all the way at the top of the stack.
367 global: &GlobalState,
368 ) -> EvalResult<'tcx> {
369 // Figure out which access `perm` corresponds to.
370 let access = if new.perm.grants(AccessKind::Write) {
375 // Now we figure out which item grants our parent (`derived_from`) this kind of access.
376 // We use that to determine where to put the new item.
377 let granting_idx = self.find_granting(access, derived_from)
378 .ok_or_else(|| InterpError::MachineError(format!(
379 "no item to reborrow for {:?} from tag {} found in borrow stack", new.perm, derived_from,
382 // Compute where to put the new item.
383 // Either way, we ensure that we insert the new item in a way such that between
384 // `derived_from` and the new one, there are only items *compatible with* `derived_from`.
385 let new_idx = if new.perm == Permission::SharedReadWrite {
386 assert!(access == AccessKind::Write, "this case only makes sense for stack-like accesses");
387 // SharedReadWrite can coexist with "existing loans", meaning they don't act like a write
388 // access. Instead of popping the stack, we insert the item at the place the stack would
389 // be popped to (i.e., we insert it above all the write-compatible items).
390 // This ensures F2b by adding the new item below any potentially existing `SharedReadOnly`.
391 self.find_first_write_incompaible(granting_idx)
393 // A "safe" reborrow for a pointer that actually expects some aliasing guarantees.
394 // Here, creating a reference actually counts as an access.
395 // This ensures F2b for `Unique`, by removing offending `SharedReadOnly`.
396 self.access(access, derived_from, global)?;
398 // We insert "as far up as possible": We know only compatible items are remaining
399 // on top of `derived_from`, and we want the new item at the top so that we
400 // get the strongest possible guarantees.
401 // This ensures U1 and F1.
405 // Put the new item there. As an optimization, deduplicate if it is equal to one of its new neighbors.
406 if self.borrows[new_idx-1] == new || self.borrows.get(new_idx) == Some(&new) {
407 // Optimization applies, done.
408 trace!("reborrow: avoiding adding redundant item {}", new);
410 trace!("reborrow: adding item {}", new);
411 self.borrows.insert(new_idx, new);
414 // Make sure that after all this, the stack's invariant is still maintained.
415 if cfg!(debug_assertions) {
416 self.test_invariants();
422 // # Stacked Borrows Core End
424 /// Map per-stack operations to higher-level per-location-range operations.
426 /// Creates new stack with initial tag.
432 let item = Item { perm: Permission::Unique, tag, protector: None };
437 stacks: RefCell::new(RangeMap::new(size, stack)),
442 /// Call `f` on every stack in the range.
447 f: impl Fn(&mut Stack, &GlobalState) -> EvalResult<'tcx>,
448 ) -> EvalResult<'tcx> {
449 let global = self.global.borrow();
450 let mut stacks = self.stacks.borrow_mut();
451 for stack in stacks.iter_mut(ptr.offset, size) {
458 /// Glue code to connect with Miri Machine Hooks
460 pub fn new_allocation(
463 kind: MemoryKind<MiriMemoryKind>,
465 let tag = match kind {
466 MemoryKind::Stack => {
467 // New unique borrow. This `Uniq` is not accessible by the program,
468 // so it will only ever be used when using the local directly (i.e.,
469 // not through a pointer). That is, whenever we directly use a local, this will pop
470 // everything else off the stack, invalidating all previous pointers,
471 // and in particular, *all* raw pointers. This subsumes the explicit
472 // `reset` which the blog post [1] says to perform when accessing a local.
474 // [1]: <https://www.ralfj.de/blog/2018/08/07/stacked-borrows.html>
475 Tag::Tagged(extra.borrow_mut().new_ptr())
481 let stack = Stacks::new(size, tag, Rc::clone(extra));
486 impl AllocationExtra<Tag> for Stacks {
488 fn memory_read<'tcx>(
489 alloc: &Allocation<Tag, Stacks>,
492 ) -> EvalResult<'tcx> {
493 trace!("read access with tag {}: {:?}, size {}", ptr.tag, ptr, size.bytes());
494 alloc.extra.for_each(ptr, size, |stack, global| {
495 stack.access(AccessKind::Read, ptr.tag, global)?;
501 fn memory_written<'tcx>(
502 alloc: &mut Allocation<Tag, Stacks>,
505 ) -> EvalResult<'tcx> {
506 trace!("write access with tag {}: {:?}, size {}", ptr.tag, ptr, size.bytes());
507 alloc.extra.for_each(ptr, size, |stack, global| {
508 stack.access(AccessKind::Write, ptr.tag, global)?;
514 fn memory_deallocated<'tcx>(
515 alloc: &mut Allocation<Tag, Stacks>,
518 ) -> EvalResult<'tcx> {
519 trace!("deallocation with tag {}: {:?}, size {}", ptr.tag, ptr, size.bytes());
520 alloc.extra.for_each(ptr, size, |stack, global| {
521 stack.dealloc(ptr.tag, global)
526 /// Retagging/reborrowing. There is some policy in here, such as which permissions
527 /// to grant for which references, and when to add protectors.
528 impl<'a, 'mir, 'tcx> EvalContextPrivExt<'a, 'mir, 'tcx> for crate::MiriEvalContext<'a, 'mir, 'tcx> {}
529 trait EvalContextPrivExt<'a, 'mir, 'tcx: 'a+'mir>: crate::MiriEvalContextExt<'a, 'mir, 'tcx> {
532 place: MPlaceTy<'tcx, Tag>,
537 ) -> EvalResult<'tcx> {
538 let this = self.eval_context_mut();
539 let protector = if protect { Some(this.frame().extra) } else { None };
540 let ptr = place.ptr.to_ptr()?;
541 trace!("reborrow: {:?} reference {} derived from {} (pointee {}): {:?}, size {}",
542 kind, new_tag, ptr.tag, place.layout.ty, ptr, size.bytes());
544 // Get the allocation. It might not be mutable, so we cannot use `get_mut`.
545 let alloc = this.memory().get(ptr.alloc_id)?;
546 alloc.check_bounds(this, ptr, size)?;
547 // Update the stacks.
548 // Make sure that raw pointers and mutable shared references are reborrowed "weak":
549 // There could be existing unique pointers reborrowed from them that should remain valid!
550 let perm = match kind {
551 RefKind::Unique { two_phase: false } => Permission::Unique,
552 RefKind::Unique { two_phase: true } => Permission::SharedReadWrite,
553 RefKind::Raw { mutable: true } => Permission::SharedReadWrite,
554 RefKind::Shared | RefKind::Raw { mutable: false } => {
555 // Shared references and *const are a whole different kind of game, the
556 // permission is not uniform across the entire range!
557 // We need a frozen-sensitive reborrow.
558 return this.visit_freeze_sensitive(place, size, |cur_ptr, size, frozen| {
559 // We are only ever `SharedReadOnly` inside the frozen bits.
560 let perm = if frozen { Permission::SharedReadOnly } else { Permission::SharedReadWrite };
561 let item = Item { perm, tag: new_tag, protector };
562 alloc.extra.for_each(cur_ptr, size, |stack, global| {
563 stack.grant(cur_ptr.tag, item, global)
568 let item = Item { perm, tag: new_tag, protector };
569 alloc.extra.for_each(ptr, size, |stack, global| {
570 stack.grant(ptr.tag, item, global)
574 /// Retags an indidual pointer, returning the retagged version.
575 /// `mutbl` can be `None` to make this a raw pointer.
578 val: ImmTy<'tcx, Tag>,
581 ) -> EvalResult<'tcx, Immediate<Tag>> {
582 let this = self.eval_context_mut();
583 // We want a place for where the ptr *points to*, so we get one.
584 let place = this.ref_to_mplace(val)?;
585 let size = this.size_and_align_of_mplace(place)?
586 .map(|(size, _)| size)
587 .unwrap_or_else(|| place.layout.size);
588 if size == Size::ZERO {
589 // Nothing to do for ZSTs.
593 // Compute new borrow.
594 let new_tag = match kind {
595 RefKind::Raw { .. } => Tag::Untagged,
596 _ => Tag::Tagged(this.memory().extra.borrow_mut().new_ptr()),
600 this.reborrow(place, size, kind, new_tag, protect)?;
601 let new_place = place.replace_tag(new_tag);
603 // Return new pointer.
604 Ok(new_place.to_ref())
608 impl<'a, 'mir, 'tcx> EvalContextExt<'a, 'mir, 'tcx> for crate::MiriEvalContext<'a, 'mir, 'tcx> {}
609 pub trait EvalContextExt<'a, 'mir, 'tcx: 'a+'mir>: crate::MiriEvalContextExt<'a, 'mir, 'tcx> {
613 place: PlaceTy<'tcx, Tag>
614 ) -> EvalResult<'tcx> {
615 let this = self.eval_context_mut();
616 // Determine mutability and whether to add a protector.
617 // Cannot use `builtin_deref` because that reports *immutable* for `Box`,
618 // making it useless.
619 fn qualify(ty: ty::Ty<'_>, kind: RetagKind) -> Option<(RefKind, bool)> {
621 // References are simple.
622 ty::Ref(_, _, MutMutable) =>
623 Some((RefKind::Unique { two_phase: kind == RetagKind::TwoPhase}, kind == RetagKind::FnEntry)),
624 ty::Ref(_, _, MutImmutable) =>
625 Some((RefKind::Shared, kind == RetagKind::FnEntry)),
626 // Raw pointers need to be enabled.
627 ty::RawPtr(tym) if kind == RetagKind::Raw =>
628 Some((RefKind::Raw { mutable: tym.mutbl == MutMutable }, false)),
629 // Boxes do not get a protector: protectors reflect that references outlive the call
630 // they were passed in to; that's just not the case for boxes.
631 ty::Adt(..) if ty.is_box() => Some((RefKind::Unique { two_phase: false }, false)),
636 // We need a visitor to visit all references. However, that requires
637 // a `MemPlace`, so we have a fast path for reference types that
638 // avoids allocating.
639 if let Some((mutbl, protector)) = qualify(place.layout.ty, kind) {
641 let val = this.read_immediate(this.place_to_op(place)?)?;
642 let val = this.retag_reference(val, mutbl, protector)?;
643 this.write_immediate(val, place)?;
646 let place = this.force_allocation(place)?;
648 let mut visitor = RetagVisitor { ecx: this, kind };
649 visitor.visit_value(place)?;
651 // The actual visitor.
652 struct RetagVisitor<'ecx, 'a, 'mir, 'tcx> {
653 ecx: &'ecx mut MiriEvalContext<'a, 'mir, 'tcx>,
656 impl<'ecx, 'a, 'mir, 'tcx>
657 MutValueVisitor<'a, 'mir, 'tcx, Evaluator<'tcx>>
659 RetagVisitor<'ecx, 'a, 'mir, 'tcx>
661 type V = MPlaceTy<'tcx, Tag>;
664 fn ecx(&mut self) -> &mut MiriEvalContext<'a, 'mir, 'tcx> {
668 // Primitives of reference type, that is the one thing we are interested in.
669 fn visit_primitive(&mut self, place: MPlaceTy<'tcx, Tag>) -> EvalResult<'tcx>
671 // Cannot use `builtin_deref` because that reports *immutable* for `Box`,
672 // making it useless.
673 if let Some((mutbl, protector)) = qualify(place.layout.ty, self.kind) {
674 let val = self.ecx.read_immediate(place.into())?;
675 let val = self.ecx.retag_reference(
680 self.ecx.write_immediate(val, place.into())?;